Finnish Estonian air ion and aerosol workshops

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1 Boreal Environment Research 12: issn Helsinki 25 June Finnish Estonian air ion and aerosol workshops Markku Kulmala 1) and Hannes Tammet 2) 1) Division of Atmospheric Sciences, Department of Physical Sciences, P.O. Box 64, FI University of Helsinki, Finland 2) Department of Environmental Physics, University of Tartu, 18 Ülikooli St., Tartu, Estonia Received 28 Dec. 2006, accepted 26 Mar (Editor in charge of this article: Veli-Matti Kerminen) Kulmala, M. & Tammet, H. 2007: Finnish Estonian air ion and aerosol workshops. Boreal Env. Res. 12: Atmospheric air ions and aerosol particles participate in atmospheric processes and have several important effects on e.g. global climate and human health. When measured, air ions and aerosol particles have been observed to be present always and in every place. In this overview we present a brief summary of the motivation, history and main achievements in air ion research. The papers in this special issue are the result of a set of bi-annual workshops taken place since The contents of the papers reflect the collaborative research efforts lead by the University of Helsinki and University of Tartu, with an increasing contribution of other partners. The main objective of these workshops has been to provide a discussion forum for the development of air-ion theory and measurement techniques. This special issue presents main achievements in a set of 14 original papers. Background and motivation Air ions exist everywhere in the Earth s atmosphere (e.g. Israël 1970, Arnold et al. 1977, Hõrrak 2001, Eichkorn et al. 2002, Lee et al. 2003). They participate in different atmospheric processes like in cloud processes and precipitation (Pruppacher and Klett 1997, Laakso et al. 2003, Andronache et al. 2006), and new aerosol particle formation (e.g. Laakso et al. 2006). In addition to direct ion studies, observations of ions offer indirect information on atmospheric aerosols and air chemistry. Observations made during the last decade or so demonstrate clearly that production of new aerosol particles by nucleation and subsequent nuclei growth is a frequent phenomenon that takes place in most atmospheric environments (e.g. Kulmala et al. 2004a). Later analyses have shown that this phenomenon is capable of affecting particle number concentrations all the way up to global scales (Spracklen et al. 2006). More regionally, atmospheric aerosol formation is likely to affect the population of particles affecting cloud properties and thereby climate (Kerminen et al. 2005, Laaksonen et al. 2005). Atmospheric aerosol formation is likely to involve multiple different pathways due to, for example, variations in the amount and type of important aerosol precursors such as low-volatile trace gases. One pathway for aerosol formation is ion-induced, or ion-mediated, nucleation. The role of ions in atmospheric aerosol formation has remained a controversial topic (Hõrrak et al. 1998, Tammet et al. 1988, Turco et al. 1998, Yu and Turco 2001, Laakso et al. 2002, Lee et al. 2003, Kazil and Lovejoy 2004, Lovejoy et at. 2004, Eisele et al. 2006, Kanawade and Tripathi 2006, Yu 2006). Some of the studies made so far, suggest that the contribution of ion-

2 238 Kulmala & Tammet Boreal Env. Res. Vol. 12 induced nucleation to the total nucleation rate is important or even dominant, whereas some other studies indicate that the role of ions in aerosol formation is negligible. The main motivation for the research presented here has been, and is, to understand atmospheric nucleation and the role of air ions play in this process. Several open questions were identified at an initial workshop in 2002: 1. How often and in how large areas new-particle formation occurs? 2. What takes place below 3 nm diameter? 3. Are there neutral clusters in the atmosphere? 4. What is the composition of nucleation mode aerosol particles? 5. What is the related thermodynamics? The close co-operation between University of Helsinki and University of Tartu started in January After that the general meetings were held nine times until March Seven of these meetings were labelled as Air ion and aerosol workshops (Table 1). The workshops were supported and organised by the Research Unit on Physics, Chemistry and Biology of Atmospheric Composition and Climate Change (Centre of Excellence, Academy of Finland) and by the Nordic centre of Excellence (BACCI: Biosphere Aerosol Cloud Climate Interactions). The workshops have acted as a forum for exchanging ideas and research results in order to address the questions mentioned above. The relevance that air-ion research has in atmospheric research is reflected in the fact that the number of participants in the workshops has doubled during the last three years. The scientific collaboration between the participants has led to about 40 published research articles both in this special issue and elsewhere, and to three master theses and one Ph.D. thesis. Instrument development The continuing collaboration between the universities (Helsinki and Tartu) and long experience gained in Tartu have resulted in the development of novel instruments for detecting ion clusters and, more recently, neutral clusters. The developed instruments have been used in a number of field experiments and laboratory studies. These instruments provide information on positive and negative ion concentrations from molecular cluster sizes up to several tens of nanometers. Apparently, the instruments give us excellent opportunity to investigate and understand atmospheric nucleation and cluster dynamics. Briefly, air ion mobility spectrometers are a class of instruments based on electric mobility analysis (e.g. Misaki 1961, Hõrrak et al. 1998, and references therein). Ion mobilities are segregated in a similarly way as is done in a DMPS (see e.g. Aalto et al. 2001), but an array of electrometers is typically used to simultaneously measure the various mobility fractions. Unlike the DMPS and SMPS systems which utilize bipolar chargers to bring the aerosol particles into Boltzmann equilibrium before being classified by the DMA, ion mobility spectrometers measure naturally occurring mobility distributions. Ion mobility spectrometers can detect Table 1. History of Finnish Estonian air ion and aerosol workshops. Name of the workshop location Date Workshop in Tartu tartu, Estonia 31 Oct Workshop in Helsinki helsinki, Finland 25 Mar First Air ion and aerosol workshop helsinki, Finland June 2003 Second Air ion and aerosol workshop helsinki, Finland Feb Third Air ion and aerosol workshop Pühajärve, Estonia 30 May 1 June 2004 Fourth Air ion and aerosol workshop helsinki, Finland Jan Fifth Air ion and aerosol workshop Pikakärve, Estonia Jan Sixth Air ion and aerosol workshop hyytiälä, Finland Mar Seventh Air ion and aerosol workshop Pühajärve, Estonia Aug Eighth Air ion and aerosol workshop hyytiälä, Finland Mar. 2007

3 Boreal Env. Res. Vol. 12 Finnish Estonian air ion and aerosol workshops 239 charged particles of any size, extending down to the size of molecular ions (ca. 0.4 nm). A limitation is that the sensitivity of electrometers limits the lowest detectable particle concentration to cm 3. The two recently-developed and already relatively widely-used instruments are the Balanced Scanning Mobility Analyzer (BSMA) (Tammet 2006) and Air Ion Spectrometer (AIS) (Mirme et al. 2007), both manufactured by AIREL Ltd., Estonia. The BSMA consists of two plain, aspiration-type differential mobility analyzers, one for positive and the other one for negative ions. The electric mobility range is cm 2 V 1 cm 1 and the ions are measured without distorting the air temperature and relative humidity inside the instrument. The mobility distribution is converted into the size distribution using a specific algorithm (Tammet 1995). The corresponding size distribution range is nm in diameter. The Air Ion Spectrometer (AIS) is measuring the mobility distribution of air ions (naturallycharged clusters and aerosol particles). The spectrometer consists of two identical cylindrical, aspiration-type differential mobility analyzers, one for measuring positive ions and the other one for measuring negative ions. The mobility range is cm 2 V 1 s 1 and the corresponding diameter range of singly-charged particles calculated according to the algorithm by Tammet (1995) is nm. Recently, the so-called Neutral cluster and Air ion Spectrometer (NAIS) was developed. Unlike the AIS, the NAIS can be used to measure neutral clusters below 2 nm diameter. The measurement principle of the NAIS is based on a unipolar charging of sampled particles and their subsequent detection with an electrical mobility analyzer. Main results A brief synopsis of main results obtained during the last few years could start from instrumental development presented in the previous section, or from mode development. The motivation behind model development was to find out the importance of ion-induced nucleation in the atmosphere (Laakso et al. 2002) Fig. 1. Sites of AIS and BSMA measurements. Points are local sites and lines are ship cruises (two) and train trips (one). and to investigate of the effect of charges on particle condensation growth (Laakso et al. 2003). Next, we calculated ion production rates a crucial parameter in ion studies based on two independent methods (Laakso et al. 2004a) and investigated different nucleation mechanisms in a boreal forest environment (Laakso et al. 2004b). At the same time, we noticed that it is possible to utilize ion spectrometers for studying sizedependent particle growth rates and concentrations of different vapours participating in particle growth processes (Kulmala et al. 2004b). We also combined data from ion spectrometers and other devices (condensation particle counters) with mode data in order to detect the potential existence of neutral molecular clusters (Kulmala et al. 2005a). We characterized cluster ion populations in forest environments (Hirsikko et al. 2005, Hõrrak et al. 2005) and utilized our measurements to calculate the charging state (a trace of new-particle formation mechanism) of nanometer-size particles (Vana et al. 2006). Recently, we studied air ion deposition processes in the atmosphere (Tammet et al. 2006), as well as the characteristics of new-particle formation based on ion measurements (Hirsikko et al. 2007b). The latest improvement from observational point of view was a measurement setup that allows us to estimate the contribution of ions to atmospheric nucleation based on direct measurement of the particle charging state (Laakso et al. 2007a). Air ion measurements were performed increasingly at different sites (Fig. 1), including in Finland, Estonia, Australia, Sweden, Hungary,

4 240 Kulmala & Tammet Boreal Env. Res. Vol. 12 Fig. 2. A typical aerosol formation and subsequent growth event (3 Apr. 2004) measured using an ion spectrometers (AIS and BSMA) in SMEAR II station, Hyytiälä, Finland. The time of day is given in the x-axis and the size (diameter) of ions is given in the y-axis. The concentration of ions (in cm 3 ) is indicated by the gray colour scale on the right. The formation of new particles starts after 09:00. The subsequent growth continues during the day and particles reach sizes of 30 nm. The growth rate can be estimated to be around 3 nm h 1. Note that the ion measurements extend down to 0.5 nm diameter, whereas typical aerosol size distribution measurements start from 3 nm. Italy, Ireland, Antarctica (Virkkula et al. 2007), high Alpine site, trip from Europe to Antarctica (using a vessel), and a train trip from Moscow to Vladivostok and back (Vartiainen et al. 2007). During those measurements, we investigated air ion dynamics in a continental boundary layer (Hirsikko et al. 2005, 2007a), marine boundary layer (Komppula et al. 2007), free troposphere (Venzac et al. 2007), coastal areas, inside clouds (Lihavainen et al. 2007), near traffic (Tiitta et al. 2007) and indoors (Hirsikko et al. 2007c). Recently, we also studied vertical profiles of air ion concentrations and aerosol formation using hot air balloon (Laakso et al. 2007b). As a summary of all our results we can state that ion clusters are always present, and that ion spectrometers make it possible to detect nucleation events very often. We also started studying processes related to water droplets during raining and snowing, in addition to which we measured air ion size distributions near a water fall (Hõrrak et al. 2005, Parts and Luts 2005, Laakso et al. 2006, Hirsikko et al. 2007b). Due to the complexity of water in its different phases, this kind of investigations will continue (e.g. Parts et al. 2007). The above investigations are promising (see also e.g. Kulmala et al. 2005a, Kulmala et al. 2007) and demonstrate the rapid progress that has been made recently in detecting thermodynamically stable clusters formed during atmospheric nucleation events and, even more interestingly, also during non-event times (e.g. Kulmala et al. 2005a). This becomes apparent when considering that already more than ten years ago Weber et al. (1995) showed that clusters were present when nm particles were detected. In Fig. 2 examples of air ion size spectra (naturally-charged cluster ions and charged aerosol particles) measured with the AIS and BSMA are presented. One of the most important features revealed by the ion instruments is that cluster ions (ions smaller than 1.5 nm) appear to be present always. This seems to be the case both during days with clear aerosol formation (event days) and during non-event days. Therefore,

5 Boreal Env. Res. Vol. 12 Finnish Estonian air ion and aerosol workshops 241 we can confidently say that there are always ion clusters in a boreal forest environment. Several measurements carried out recently in different locations (marine atmosphere, urban air, free troposphere, coastal areas) confirm this observation. The other important feature seen from Fig. 2 is that atmospheric nucleation seems to start from sizes (mass or Tammet s diameter) of around 1.5 nm. Another important observation is that after new-particle formation, ion measurements show particle growth patterns ( banana-shaped ) similar to those shown by DMPS measurements. According to a recent overview (Kulmala et al. 2004a), the measured growth rate of nucleation mode particles is typically remarkably constant above the sizes of 5 8 nm. In our own stations, atmospheric nucleation is seen to be frequent and important (e.g. Dal Maso et al. 2005, Pugatsova et al. 2007). From the point of view of cluster dynamics and especially new-particle formation, the chemical composition of clusters is important (e.g. Mäkelä 1992). In the continuously-ionized atmosphere, the mobility spectrum of natural small ions is a mixture of ions of different age and chemical composition, evolving via ionmolecule reactions (e.g. Mohnen 1977, Luts 1995, Beig and Brasseur 2000). The composition of cluster ions depends strongly on their age and concentrations of trace species in the air, as well as on the thermodynamic properties of the clusters and surrounding air (e.g. Bricard et al. 1972, Castleman et al. 1978, Eisele and Tanner 1990, Nagato and Ogawa 1998, Parts and Luts 2004). To summarise and conclude, the studies presented above and in the following articles give the following answers to our initial set of questions (see above): 1. New-particle formation seems to occur over large areas, being an important phenomenon (e.g. Kulmala et al. 2004a). However, more air ion and particularly neutral cluster measurements are still needed. 2. Ion spectrometers can show some part of the evolution and dynamics of small ion clusters. Therefore we have developed several ion spectrometers. 3. Ion clusters are anyhow present, in practice everywhere. Model calculations suggest that there should also be neutral clusters (Kulmala et al. 2000). There is increasing evidence on the existence of neutral clusters (e.g. Kulmala et al. 2005a), and we have developed new instruments for detecting them. 4. Recently we have developed new in situ methods like the CPC battery for studying the water solubility of very small particles (Kulmala et al. 2007) 5. Although we know that the saturation concentration of condensing vapours seems to be around cm 3, a deep insight on the thermodynamical properties of atmospheric clusters is still missing. Future perspectives Although our extensive investigations have given a lot of new insight on air ion dynamics and fresh aerosol particle formation, there are still, even increasing, number of open questions. In future we will work with these research questions using both theoretical and experimental approaches. We will focus on the following issues: 1. Continuous air ion cluster and nucleation mode aerosol particle measurements at the SMEAR II station in Hyytiälä and simultaneous air ion measurements in Estonia will continue. The routine measurements of air ion mobility distributions in Tahkuse, Estonia, have been running since We started ion measurements in Hyytiälä in spring Currently we are measuring atmospheric ions with several devices: Air Ion Spectrometer (AIS, nm), Balanced Scanning Mobility Analyzer (BSMA, nm), Neutral cluster Air Ion Spectrometer (NAIS, ions: nm, neutral particles: nm) and ion-differential Mobility Particles Spectrometer (ion-dmps, the charging state of 3 15 nm particles). In addition, we are measuring size distributions of nm particles with a normal DMPS. In future we will decrease the cut size of both normal and ion-dmps systems toward 2 nm. 2. Since the NAIS is a prototype version of a new measurement device, we will con-

6 242 Kulmala & Tammet Boreal Env. Res. Vol. 12 tinue its development together with University of Tartu and AIREL Ltd. The NAIS and ion spectrometer will be calibrated and tested under laboratory conditions and used in different laboratory experiments in order to obtain data under well-defined conditions. The ion spectrometers are relative new devices, so we continue their development (Mirme et al. 2007). However, this will take several years. We will also continue our laboratory experiments with the AIS and NAIS. 3. World-wide measurements will be conducted as campaign-wise measurements. With the ion spectrometers and NAIS s, we can detect atmospheric nucleation (both neutral and ion induced nucleation or activation) and its importance in different regions. Currently, we have measured ion size distributions in Finland, Estonia, Atlantic Ocean, Siberia, Arctic regions, boreal forest, urban environment, South-African savannah, coastal environment at Mace Head, Ireland, Pusta in Hungary, the Alps, Antarctica and Australia. Preliminary results from these environments emphasize the importance of ion-induced nucleation, especially under cold and clean conditions. 4. The data set we have collected is unique and subsequent data analysis will take several years. With this data, we will calculate (a) the contribution of ion-induced nucleation to total atmospheric nucleation, (b) concentrations and source rates of condensable vapours (see Kulmala et al. 2001, Kulmala et al. 2005b, Dal Maso et al. 2005), and (c) ion characteristics and their differences in various environments. As a part of these studies, we will develop theoretical methods for the data analysis of ion spectrometers and ion-dmps. 5. Our recent observations have shown that there are several unknown ion phenomena related to water droplets and ice crystals (Hirsikko et al. 2007b). Based on our extensive data set and laboratory experiments, we will study these phenomena in more detail. These processes include the formation of small intermediate ions in the presence of ice crystals and the formation of nanometer-size water droplets from rain droplets and waterfalls. 6. Our measurements have shown some inconsistency between the predicted and measured air ion generation rates in a boreal forest (Laakso et al. 2004a, Tammet et al. 2006, Hirsikko et al. 2007a). We will develop the methods of air ion generation measurement and the air ion balance models with aim to achieve an adequate description of air ion dynamics in a boreal forest. This includes consideration of the height profiles inside the forest canopy. 7. One of unknown phenomena related to climate change is the possible interaction between solar activity and ion-induced nucleation in the atmosphere (e.g. Svensmark and Friis-Christensen 1997). We will study processes related to cosmic ray-climate interactions in an aerosol and cloud chamber in CERN, Switzerland, in a project called CLOUD. 8. We will continue to develop basic theories related to ion processes (e.g. Noppel et al. 2003, Lushnikov and Kulmala 2004a, 2004b, Lushnikov and Kulmala 2005), and to apply quantum chemistry models (Kurten et al. 2007). The basic theories related to different hypothesis made for cluster dynamics and nucleation processes will be tested and further developed (e.g. Kulmala et al. 2006). Since ion spectrometers measure clusters consisting of few molecules, we will investigate air chemistry by combining ion measurements and molecular modeling (Bonn et al. 2006). We will also look in more detail the possibilities to utilize time-of-flight-mass spectrometers for the chemical analysis of clusters and air chemistry. 9. During last years, we have developed a complete ion process model AEROION (Laakso et al. 2002). We will continue developing this model as a part of the UHMA model family (Korhonen et al. 2004) to find out the importance off air ions in different aerosol dynamic processes. Ultimately, we aim to parameterize the results for global climate models. Acknowledgements: Financial support from the Academy of Finland, Estonian Science Foundation and particularly from Nordic centre of Excellence (BACCI) is acknowledged. We are thankful for encouraging discussions with and help

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9 Boreal Env. Res. Vol. 12 Finnish Estonian air ion and aerosol workshops 245 Conference on Atmospheric Electricity 1, Versailles, pp Parts T.-E., & Luts A Observed and simulated effects of certain pollutants on small air ion spectra: I. Positive ions. Atmos. Environ. 38: Parts T.-E. & Luts A Potential new air ions from waterfall particles. In: Maenhaut W. (ed.), Abstracts of the European Aerosol Conference 2005, Ghent, Belgium, 28 August 2 September 2005, p Parts T.-E., Luts A., Laakso L., Hirsikko A., Grönholm T. & Kulmala M Chemical composition of waterfallinduced air ions: spectrometry vs. simulations. Boreal Env. Res. 12: Pruppacher H.R. & Klett J.D Microphysics of clouds and precipitation. Second revised and enlarged edition with an introduction to cloud chemistry and cloud electricity. Springer Verlag. Pugatšova A., Iher H. & Tamm E Modal structure of the atmospheric aerosol particle size spectrum for nucleation burst days in Estonia. Boreal Env. Res. 12: Reischl G.P., Mäkelä J.M., Karch R. & Necid J Bipolar charging of ultrafine particles in the size range below 10 nm. J. Aerosol Sci. 27: Spracklen D.V., Carslaw K.S., Kulmala M., Kerminen V.- M., Mann G.W. & Sihto S.-L The contribution of boundary layer nucleation events to total particle concentrations on regional and global scales. Atmos. Chem. Phys. 6: Svensmark H. & Friis-Christensen E Variation in cosmic ray flux and global cloud coverage a missing link in solar-climate relationships. J. Atmos. Solar-Terr. Phys. 59: Tammet H Size and mobility of nanometer particles, clusters and ions. J. Aerosol Sci. 26: Tammet H Continuous scanning of the mobility and size distribution of charged clusters and nanometer particles in atmospheric air and the Balanced Scanning Mobility Analyzer BSMA. Atmos. Res. 82: Tammet H. & Kulmala M Simulation tool for atmospheric aerosol nucleation bursts. J. Aerosol Sci. 36: Tammet H. & Kulmala M Simulating aerosol nucleation bursts in a coniferous forest. Boreal Env. Res. 12: Tammet H., Salm J. & Iher H Observation of condensation on small air ions in the atmosphere. Lect. Notes Phys. 309: Tammet H., Hõrrak U., Laakso L. & Kulmala M Factors of air ion balance in a coniferous forest according to measurements in Hyytiälä, Finland. Atmos. Chem. Phys. 6: Tiitta P., Miettinen P., Vaattovaara P., Laaksonen A., Joutsensaari J., Hirsikko A., Aalto P. & Kulmala M Road-side measurements of aerosol and ion number size distributions: a comparison with remote site measurements. Boreal Env. Res. 12: Turco R.P., Zhao J.-X. & Yu F A new source of tropospheric aerosols: ion-ion recombination. Geophys. Res. Lett. 25: Vana M., Kulmala M., Dal Maso M., Hõrrak U. & Tamm E Comparative study of nucleation mode aerosol particles and intermediate air ions formation events at three sites. J. Geophys. Res. 109, D17201, doi: / 2003JD Vana M., Tamm E., Hõrrak U., Mirme A., Tammet H., Laakso L., Aalto P.P. & Kulmala M Charging state of atmospheric nanoparticles during the nucleation burst events. Atmos. Res. 82: Vartiainen E., Kulmala M., Ehn M., Hirsikko A., Junninen H., Petäjä T., Sogacheva L., Kuokka S., Hillamo R., Skorokhod A., Belikov I., Elansky N. & Kerminen V.-M Ion and particle number concentrations and size distributions along the Trans-Siberian railroad. Boreal Env. Res. 12: Venzac H., Sellegri K. & Laj P Nucleation events detected at the high altitude site of the Puy de Dôme Research Station, France. Boreal Env. Res. 12: Virkkula A., Hirsikko A., Vana M., Aalto P.P., Hillamo R. & Kulmala M Charged particle size distributions and analysis of particle formation events at the Finnish Antarctic research station Aboa. Boreal Env. Res. 12: Weber R.J., McMurry P.H., Eisele F.L. & Tanner J Measurement of expected nucleation precursor species and nm diameter partciles at Mauna Loa observatory, Hawaii. J. Atmos. Sci. 52: Wilding R.J. & Harrison R.G Aerosol modulation of small ion growth in coastal air. Atmos. Environ. 39: Yu F From molecular clusters to nanoparticles: Second-generation ion-mediated nucleation model. Atmos. Chem. Phys. 6: Yu F. & Turco R.P From molecular clusters to nanoparticles: Role of ambient ionization in tropospheric aerosol formation. J. Geophys. Res. 106: